Light-Effect Transistor (LET)

ABSTRACT

Example photoconductive devices and example methods for using photoconductive devices are described. An example method may include providing a photoconductive device having a metal-semiconductor-metal structure. The method may also include controlling, based on a first input state, illumination of the photoconductive device by a first optical beam during a time period, and controlling, based on a second input state, illumination of the photoconductive device by a second optical beam during the time period. Further, the method may include detecting an amount of current produced by the photoconductive device during the time period, and based on the detected amount of current, providing an output indicative of the first input state and the second input state. The example devices can be used individually as discrete components or in integrated circuits for memory or logic applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims the benefit of U.S. Provisional Application No.62/238,781 filed on Oct. 8, 2015, the entirety of which is hereinincorporated by reference.

FIELD

The present disclosure relates generally to electronic andoptoelectronic devices, and more particularly, to a particular type ofphotoconductive device, referred to as a light-effect transistor (LET),and to methods for using a LET to perform optical gating and opticalanalog operations.

BACKGROUND

As basic electronics building blocks, field-effect transistor (FETs) arewidely used in both logic and memory chips. A typical FET is athree-terminal device consisting of source (S), drain (D), and gate (G)contacts. In operation, conductivity between the S and D contacts ismodulated by applying a voltage or an applied electric field through theG contact, resulting in ON and OFF states. Although FETs have evolvedstructurally from early planar geometries to their currentthree-dimensional (3D) geometries and have continually shrunk in size,the basic operating principal remains the same. The structural changesand shrinkage have led to ever greater fabrication complexity, andultimately to challenges in gate fabrication and doping control.

Various new technologies, such as fin field-effect transistors (FinFETs)and tunnel-FETs have been developed in recent years to enable thecontinuation of Moore's law. Other types of FETs are also being exploredas alternatives, such as semiconductor nanowire (SNW) based FETs, FETscomprised of two-dimensional (2D) materials, and FETs with multipleindependent gates or gates with embedded ferroelectric material.However, the above-referenced technologies may not provide a clearpathway for eliminating gating complexity or avoiding difficulties withdoping control. Furthermore, a viable rival technology does notcurrently exist.

SUMMARY

In one example, an optical gating method is described. The methodincludes providing a photoconductive device having ametal-semiconductor-metal structure. The method also includescontrolling, based on a first input state, illumination of thephotoconductive device by a first optical beam during a time period andcontrolling, based on a second input state, illumination of thephotoconductive device by a second optical beam during the time period.In addition, the method includes detecting an amount of current producedby the photoconductive device during the time period. And the methodincludes, based on the detected amount of current, providing an outputindicative of the first input state and the second input state.

In another example, a method for optical amplification is described. Themethod includes providing a photoconductive device having ametal-semiconductor-metal structure. The method also includesilluminating the photoconductive device using an optical beam whilesimultaneously exposing the photoconductive device to an optical signalduring a time period. Further, the method includes detecting an amountof current produced by the photoconductive device during the timeperiod. Illuminating the photoconductive device using the optical beammay amplify the amount of current induced by the optical signal.

In still another example, another optical gating method is described.The method includes providing a photoconductive device including twometal contacts separated by a semiconductor nanostructure channel. Themethod also includes controlling, based on a first input state,illumination of the semiconductor nanostructure channel by an opticalbeam during a time period and controlling, based on a second inputstate, a bias voltage applied across the two metal contacts during thetime period. In addition, the method includes detecting an amount ofcurrent produced by the photoconductive device during the time period.And the method includes, based on the detected amount of current,providing an output indicative of the first input state and the secondinput state.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill be become apparent by reference to the following detaileddescription and accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B are a schematic comparison between an example FET and anexample LET.

FIGS. 2A and 2B are images of an example LET.

FIGS. 2C and 2D illustrate characteristics of the example LET of FIGS.2A and 2B.

FIGS. 3A-3F illustrate output characteristics of two example LETs.

FIGS. 4A-4F illustrate transfer characteristics of the two example LETsdiscussed with reference to FIGS. 3A-3F.

FIGS. 5A-5H illustrate example functionalities of the example LETdiscussed with reference to FIGS. 3E-3F.

FIGS. 6A-6D illustrate example optical logic gate constructions andtruth tables.

FIGS. 7A-7D illustrate additional example optical logic gateconstructions and truth tables.

FIG. 8 is a flow chart of an example optical gating method, according toan example embodiment.

FIG. 9 is a flow chart of another example optical gating method,according to an example embodiment.

FIG. 10 is a flow chart of an example method for optical amplification.

FIG. 11 is a schematic diagram of an example computing device accordingto an example embodiment.

DETAILED DESCRIPTION

Example systems and methods are described herein. Any example embodimentor feature described herein is not necessarily to be construed aspreferred or advantageous over other embodiments or features. Theexample embodiments described herein are not meant to be limiting. Itwill be readily understood that certain aspects of the disclosed systemsand methods can be arranged and combined in a wide variety of differentconfigurations, all of which are contemplated herein.

Furthermore, the particular arrangements shown in the Figures should notbe viewed as limiting. It should be understood that other embodimentsmight include more or less of each element shown in a given Figure.Further, some of the illustrated elements may be combined or omitted.Yet further, an example embodiment may include elements that are notillustrated in the figures.

As discussed above, this disclosure relates to a particular type ofphotoconductive device referred to as a LET. A LET has ametal-semiconductor-metal structure, with one of the metal-semiconductorjunctions serving as an S contact and the other one of themetal-semiconductor junctions serving as a D contact. The S and Dcontact are separated by a semiconductor channel (e.g., a semiconductornanostructure channel). Sometimes, a thin insulating layer can be addedor may naturally exist between the S contact and the semiconductorchannel and/or between the D contact and the semiconductor channel toachieve further device tuning. In operation, one or multipleindependently controlled optical beams may be applied to thesemiconductor channel to modulate the conductivity between the S and Dcontacts. In other words, a LET employs optical gating, which contrastsa FET's electrostatic control through an applied voltage.

As discussed more fully below, LETs exhibit electrical and opticalcharacteristics that can replicate the basic switching function of a FETand also provide new functionalities not readily achievable by a FET.Advantageously, the simplistic architecture of a LET eliminates theabove-referenced gate fabrication complexity and doping controlchallenges associated with FETs.

Within examples, methods for using a LET to implement electrical andoptical hybrid logic gates are provided. In one example, a bias voltagemay be applied across the S and D contacts of a LET while modulatingillumination of the LET with a single beam. When the bias voltage isapplied, illuminating the LET by the single beam induces a detectable,threshold high amount of current between the S and D contacts. Whereas,illuminating the LET without applying the bias voltage or applying thebias voltage without illuminating the LET does not induce as muchcurrent between the S and D contacts. Accordingly, a detector canmeasure the amount of current between the S and D contacts and then,based on the measured amount of current, provide an output indicative ofwhether the bias voltage is applied and the LET is illuminated. Hence,under single beam illumination, the FET can function as a hybrid ANDlogic gate. This basic operation mode can be used in differentsituations. For instance, this basic operation mode can be used toprovide a logic switch in an integrated circuit containing a pluralityof LETs, where each LET is nanoscale in size. Or this basic operationmode can be used to provide the function of an insulated-gate bipolartransistor (IGBT) in a power electronics device that is larger in sizeand handles large electrical signals.

Further, a pair of LETs can be combined, with each LET of the pair beingcontrolled by a respective optical beam, to realize a universal logicgate (e.g., a NOR or NAND logic gate). Such universal logic gates can beused to build complex integrated logic circuits with high speed and lowenergy advantages as compared to FET-based technologies.

In other examples, a computing device such as a logic device mayindependently control illumination of a LET by at least two beams inorder to implement an optical logic gate. For instance, the computingdevice may control, based on a first input state, illumination of theLET by a first optical beam during a time period and also control, basedon a second input state, illumination of the LET by a second opticalbeam during the time period. A detector may then detect an amount ofcurrent produced by the LET during the time period. Based on thedetected amount of current, the computing device may provide an outputindicative of the first input state and the second input state. Forinstance, the output may be indicative of a logical combination of thefirst input state and the second input state.

In one instance, with a constant bias voltage applied across the S and Dcontacts, the two illuminating optical beams can result in optical ANDor optical OR logic gates, depending on the operation point on the LET'stransfer curve. In another instance, by controlling the bias voltage,logic gates with three inputs can be achieved.

Performing optical amplification using a LET may involve illuminatingthe LET using an optical beam while simultaneously exposing the LET toan optical signal during a time period. For instance, the LET may beilluminated with the optical beam while simultaneously exposing the LETto an infrared signal in an environment. Further, a bias voltage may beapplied across two metal contacts of the LET. A computing device (e.g.,a current meter) may then detect an amount of current produced by theLET during the time period. Advantageously, illuminating the LET usingthe optical beam may amplify the amount of current induced by theoptical signal by a factor of at least ten. Hence, with this approach, aLET may be used for weak optical signal (e.g., infrared signal)detection.

Various example implementations of these methods are described belowwith reference to the figures.

Referring now to the Figures, FIGS. 1A and 1B are a schematic comparisonbetween a FET 100 and an example LET 120. As shown in FIG. 1A, the FET100 includes an S contact 102, a D contact 104, and a G contact 106. Asemiconductor nanostructure channel 108 connects the S contact 102 andthe D contact 104. The S contact 102, D contact 104, G contact 106, andsemiconductor channel 108 all overlay an insulating dielectric 110.Further, the insulating dielectric 110 rests upon a substrate 112.

In one example, the S contact 102, D contact 104, and G contact 106 mayeach be metal contacts, and the semiconductor nanostructure channel 108may be a semiconductor nanowire. Further, the insulating dielectric 110may be a silicon oxide layer and the substrate 112 may be a siliconsubstrate.

In operation, S-D current I_(ds) between the S contact 102 and the Dcontact 104 through the semiconductor nanostructure channel 108 isdriven by an S-D voltage V_(ds) and modulated through a gate voltageV_(g). Current carriers are generated by thermal activation of dopants.

Similarly, as shown in FIG. 1B, the LET 120 includes an S contact 122, aD contact 124, and a semiconductor nanostructure channel 126 connectingthe S contact 122 and the D contact 124. Unlike the FET 100, however,the LET 120 does not include a G contact—no physical gate is required.The S contact 122, D contact 124, and semiconductor nanostructurechannel 126 all overlay an insulating dielectric 128. Further, theinsulating dielectric 128 rests upon a substrate 130.

In one example, the S contact 122 and D contact 124 may each be metalcontacts, and the semiconductor nanostructure channel 126 may be asemiconductor nanowire. Further, the insulating dielectric 128 may be asilicon oxide layer and the substrate 130 may be a silicon substrate. Inother examples, LETs may have larger size semiconductor channels. Hence,the semiconductor channel is not limited to nanoscale channels.

The operation mechanism of the LET 120 is distinctly different from theoperation mechanism of the FET 100 in two regards. First, S-D currentI_(ds) between the S contact 122 and the D contact 124 is modulated bylight or an optical frequency electromagnetic field P_(g)(λ_(g)), whichcontrasts the control of the S-D current of the FET 100 through the gatevoltage V_(g). Second, in the LET 120, current carriers are generatedthrough optical absorption rather than by thermal activation of dopants.In other words, the LET 120 uses optical gating based uponphotoconductivity.

Under one-beam continuous wave operation, optical gating throughP_(g)(λ_(g)) has two basic control parameters: wavelength λ_(g) andpower level P_(g). Optical gating can be extended to other operationmodes. For instance, multiple independent beams and pulsed illuminationmay be represented as P_(g)(λ_(g1), λ_(g2), . . . , λ_(gN)) andP_(g)┌(t₁,λ_(g1)), (t₂,λ_(g2)), . . . , (t_(N)λ_(gN))┐, respectively.

Because of the simplistic architecture of the LET 120 as compared to thearchitecture of the FET 100, the LET 120 provides several advantagesover the FET 100. For example, the LET 120 eliminates gate fabricationcomplexity and avoids difficulties with doping control. As such, the LET120 does not suffer from the two primary challenges faced by the FET 100when attempting to downscale to the quantum regime. Furthermore, the LET120 offers the potential for reduced fabrication costs as compared withthe fabrication costs of the FET 100. Also, while the most basicapplication of the LET 120 emulates the FET 100 when it operates underone-beam illumination, the LET 120 offers functionalities not readilyachievable by the FET 100, such as when responding to multipleindependently controlled light beams. In addition, the frequencyresponse of the LET 120 is limited by the carrier transit time throughits conducting channel, while the frequency response of the FET 100 islimited by its gate capacitance. Thus, LETs can potentially operate atmuch higher frequencies, up to terahertz frequencies and beyond, andconsequently can operate with far lower switching energies.

Light-induced electrical conductivity changes are typically used forphoto-detection. However, photo-detectors may lack desirable FET-likecharacteristics and are therefore unsuitable for LET use or have notbeen characterized using the metrics relevant to a FET's switchingapplication. Photo-detection typically relies upon a p-n junction-baseddevice rather than a metal-semiconductor-metal device becausemetal-semiconductor-metal devices typically require a larger biasvoltage to drive carriers. In addition, a p-n junction basedphoto-detector has a distinctly different current-voltage (I-V)characteristic under illumination than a metal-semiconductor-metaldevice—as the latter can offer an I-V characteristic resembling that ofa FET with gate voltage on. Further, under simultaneous multi-beamillumination, which is usually irrelevant for photo-detection, themultiple independent gating capability enables a LET to demonstratepreviously unreported functionalities, such as optical logic (e.g., ANDand OR) gates and optical amplification as an analog application.

LETs are also capable of quantum scale operation. A LET shares the samelimit of a FET, that is, nanostructure dimensions are practicallyachievable, e.g., 1-7 nm for Si nanowires. However, LETs do not requirecomplex and sophisticated fabrication steps for physical gates anddoping. In general, ballistic transport theory suggests thatcommercially viable currents could be achieved in quantum structures.The on-state energy consumption could be as low as ˜13 nW/LET whenI_(ds)=1 μA, and the required minimal V_(ds) would only be 13 mV (notincluding the voltage drop over the semiconductor-metal junctions), whena minimum quantum conductance is assumed. Thus, LETs could operate withlow energy consumption.

At least two basic illumination modes for LETs are possible, dependingon the application: (i) uniform, broad-area illumination over ahigh-density LET array, or (ii) separated light beams directed toindividual or small groups of LETs through, for instance, sharp fibertips or nanoscale emitters embedded on the same chip. For either mode,multiple light sources of the same or different wavelength(s) and/orintensities can be combined into one beam but controlled independently.

FIGS. 2A and 2B are images of an example LET. In particular, FIGS. 2Aand 2B are images of a LET having a cadmium selenide (CdSe) nanowire andindium contacts. The CdSe nanowire was grown in a vertical array throughgold-catalyzed chemical vapor deposition, and then dispersed in alcoholand drop cast onto a silicon/silicon oxide (Si/SiO₂) chip. The Si/SiO₂chip consisted of an Si substrate coated with a 300-nm thick SiO₂ layer.After the CdSe nanowire was dispersed onto the chip, a thin poly-methylmethacrylate (PMMA) layer was spin coated onto the chip, followed byelectron-beam lithography to open channels at the nanowire's ends.Exposed PMMA was removed by developing the chip. Afterwards, the chipwas transferred to a thermal evaporator for indium metallization,followed by lift-off in acetone to obtain a finished device. In otherexamples, an array of nanowires can be grown on a patterned substrate,and processed into devices in parallel (not shown). Hence, the exampleimages are not meant to be limiting.

FIG. 2A is a scanning electron microscope (SEM) image of a 10-μm longCdSe semiconductor nanowire (SNW) device with indium contacts formingmetal-semiconductor (M-S) junctions at each end. The scale bar 200 inFIG. 2A is a 2-μm scale bar.

FIG. 2B is a low magnification transmission electron microscopy (TEM)image. As revealed by the low magnification TEM image of FIG. 2B, theCdSe SNW is a uniform single-crystalline CdSe SNW grown in wurtzitephase along the [0001] axis with a diameter of ˜80 nm. The scale bar 202in FIG. 2B is a 100-nm scale bar. The inset 204 in the image of FIG. 2Bis a selected area electron diffraction (SAED) pattern.

FIG. 2C depicts the CdSe SNW's laser-power-dependent photoluminescence(PL) obtained under 442-nm excitation at different powers, with P₀ equalto 1.5 μW. The PL shows a strong emission peak at 1.78 eV that matchesCdSe's bandgap energy. The inset 206 overlays a PL map upon an opticalimage, demonstrating relatively homogenous SNW emission, and byextension, homogenous material quality across the SNW channel. The scalebar 208 in the inset 208 is a 4-μm scale bar.

FIG. 2D depicts the output characteristic I_(ds) vs. V_(ds) for the CdSeSNW device, with and without illumination using a halogen light. Theillumination optically modulates or “gates” the electrical conductivitybetween OFF and ON states. The first curve 210 represents I_(ds) withoutillumination, and the second curve 212 represents I_(ds) withillumination. The first and second curves 212, 214 resemble those of aFET's OFF and ON states, respectively, especially when V_(ds)<˜7 V.

FIGS. 3A-3F illustrate output characteristics of two example LETs. Inparticular, FIGS. 3A-3F illustrate S-D current I_(ds) as a function ofthe applied source-drain voltage V_(ds) with varying gate power P₀ andwavelength λ_(g) for two devices, device 1 (D1) and device 2 (D2). D1and D2 have CdSe nanowires with lengths of ˜10 μm and ˜5.5 μm,respectively, and diameters of ˜80 nm. Representative characteristicsare shown in FIGS. 3A-3D for D1 and in FIGS. 3E-3F for D2. The P₀ valuesfor FIGS. 3A-3D are 1.40, 2.07, 2.38, and 2.25 μW, respectively. The P₀values for FIGS. 3E-3F are 1.38 and 69.1 μW, respectively. The darkcurrent (no illumination) is represented in FIGS. 3A-3F as solid blacklines.

The characteristics exemplify how LET performance depends on the gatepower/wavelength, illumination condition, and device variation. By wayof example, D1 exhibits two well-separated plateaus, respectively,starting at V_(ds)˜4-5 V and ˜14-18 V depending on the gate wavelengthand power. For example, the second plateau's onset is at ˜14-15 V for633 nm illumination but shifts to ˜16-18 V under 442-nm excitation. Twotunable plateaus can potentially offer two distinct, customizable ONstates (e.g., a low-ON state and a high-ON state).

For D2, the first and second plateaus are comparatively not wellseparated, and both 532-nm and halogen illumination have their firstplateau at ˜2 V with respective power-dependent, second plateaus at˜6-7.5 V (532 nm) and ˜5-5.75 V (halogen). Each plateau appears atrespectively lower V_(ds) values than in D1. Because of the extremelylow dark current, the long second plateau extends to the highest V_(ds)measured.

For D1, the maximum on/off ratios typically occur at V_(ds)<5 V, andvary from 10² to 10⁴ depending on the gate power and wavelength. Forinstance, FIG. 3B shows on/off ratios of ˜5×10⁴ and ˜2×10⁴ atV_(ds)=1.43 V and 4.95 V, respectively, when P_(g)(532 nm)≈2 μW. FIG. 3Eshows on/off ratios for D2 of ˜1.0×10⁶ and ˜1.1×10⁶ at V_(ds)=1.43 and4.95 V, respectively, when P_(g)(532 nm)≈2.6 μW. FIG. 3F shows on/offratios for D2 of ˜6×10⁵ and ˜1×10⁶ at V_(ds)=1.43 and 4.95 V,respectively, when P_(g)(halogen)≈69 μW.

Differences between D1 and D2 indicate that a LET's characteristics maybe tuned and optimized through material and device engineering. A largemetal-SW (M-SNW) contact barrier may be desirable for producing smallOFF state currents over the operation range, and can be optimized tomaximize the on/off ratio. Current levels for different “gate”wavelengths in FIGS. 3A-3D show considerable variations, which isfundamentally due to wavelength-dependent light-matter interactioneffects, e.g., absorption and carrier dynamics, and illuminationconditions, e.g., power density and beam size. This feature offersgreater flexibility in achieving gate functions as compared with theflexibility in achieving gate functions with FETs.

FIGS. 4A-4F illustrate transfer characteristics of D1 and D2. Inparticular, FIGS. 4A-4F illustrates S-D current I_(ds) as a function oflaser power under different source-drain voltages V_(ds). Representativecharacteristics are shown in FIGS. 4A-4D for D1 and in FIGS. 4E-4F forD2.

The transfer characteristics allow extraction of several performancemetrics. A FET's threshold gate voltage V_(T) and subthreshold swing Sare respectively defined as the onset of a linear region in theI_(ds)−V_(g) curve (i.e., voltage-controlled resistor behavior), and asthe inverse linear slope on a semi-log I_(ds)−V_(g) plot. Their physicalinterpretations, respectively, are the gate voltage required for deviceoperation and the gate voltage increment to induce an order of magnitudecurrent change below V_(T). A small S value implies a small energy orpower consumption to turn on or operate a FET.

The transfer characteristics for D1 and D2 generally resemble a FET'stransfer characteristics, e.g., increasing I_(ds) as the gate powerP_(g) increases under constant V_(ds), except a LET replaces V_(g) withP_(g). A LET's threshold gate power P_(T), then corresponds to the onsetof a linear I_(ds)−P_(g) region for a given λ_(g), and S_(TET) is itssubthreshold swing. Significantly, FETs usually do not operate in thesubthreshold swing region, while a LET can employ this range to realizeoptical logic gates and for an interesting optical amplification effect.

FIG. 4E, for example, shows that, with λ_(g)=532 nm, P_(T) and S_(TET)are ˜30 nW and ˜1.8 nW/decade, respectively. For reference, advancedFETs have respective V_(T) and S parameters of 100-200 mV and ˜70-90mV/decade.

At V_(ds)=1.43 V, P_(g)=0.11 μW yields an I_(ds) of 0.35 μA, and a LETdynamic power consumption of ˜0.5 μW, which is comparable to advancedFETs. A LET's OFF state energy consumption can be very low. Forinstance, the dark current is ˜1 pA at V_(ds)=1.43 V with acorresponding OFF power consumption of ˜1.5 pW, which is lower than aFET of similar length.

The switching energy, or the amount of energy needed to go from OFF toON states, of a LET can be lower than the switching energies of modernFETs. A LET can be viewed as a FET without the gate, which means that aLET's switching time is limited by the carrier transit time rather thana FET's capacitive delay. Most direct band gap semiconductors possessroom temperature carrier lifetimes on the order of 100 ps without anapplied bias, where applying a bias, especially for a short conductingchannel length, reduces the transit time by more quickly wiping out freecarriers. Simple estimates based on D2's performance support lowerswitching energies in LETs. For example, even an assumed 100 ps delaytime would yield a switching energy of 0.05 fJ/switch (0.5 μW×100 ps)associated with the S-D current, and 0.01 fJ/switch (0.1 μW×100 ps) fromthe optical gating action, which yields a total switching energy lessthan typical FET values of 0.1-1.0 fJ/switch. The switching energy couldbe further reduced by reducing the channel length and optimizing thecontacts.

The representative LET transfer characteristics in FIGS. 4A-4F can beused to illustrate the underlying principles for a few differentapplications. For example, FIGS. 5A-5H illustrate examplefunctionalities using D2. In FIG. 5A, D2's 532-nm illuminationcharacteristics (from FIG. 4E) are re-plotted on a double log scale. Forclarity, only the curves for V_(ds)=1.43 and 4.98 V are shown. FIG. 5Ashows three major operating regions: a super-linear region 500, a linearregion 502, and a sublinear/saturation region 504. The applied gatingpower determines the operating region. For instance, in the example ofFIG. 5A, D2 can be operated in the super-linear region 500 by applying agating power of <0.010 μW, operated in the linear region 502 by applyinga gating power of 0.1 μW, and operated in the sublinear/saturationregion 504 by applying a gating power >1 μW.

Each of the different regions can offer different unique applications.By way of example, FIG. 5B demonstrates single-beam illumination as ahybrid AND logic gate, which replicates the most basic FET logicfunction. An electrical input A=V_(ds) and an optical input B=P_(g) caneach be controlled, and D2 can provide an output denoted as A×B. Asshown in FIG. 5B, when either A or B is not applied (e.g., when V_(ds)=0V or P_(g)=0 μW), D2 produces a threshold low I_(ds). However, when bothA and B are applied (e.g., when V_(ds)>a threshold V_(ds) and P_(g)>athreshold P_(g)), D2 produces a threshold high I_(ds).

Accordingly, when illumination of D2 is controlled based on a firstinput state (e.g., illuminating D2 if the first input state is an ONstate but not illuminating D2 if the first input state is an OFF state)and the source-drain voltage is controlled based on a second input state(e.g., applying a source-drain voltage if the second input state is anON state but not applying a source-drain voltage if the second input isan OFF state), the amount of source-drain current can be interpreted todetermine whether both the first input state and the second input stateare ON input states. For instance, detecting a threshold high amount ofsource-drain current can be interpreted to mean that both the firstinput state and the second input state are ON states. Whereas, detectinga threshold low amount of source-drain current can be interpreted tomean that one or both of the first input state and second input statesare OFF states.

One-beam operation could also act as a current source or voltageamplifier when operating in the output characteristic'ssublinear/saturation region 504. Further, a LET's two distinct ON states(e.g., the first and second plateaus in FIG. 3B) can be utilized torealize two-level logic gate and voltage amplifier functions. By way ofexample, applying any level of source-drain voltage without anyillumination may induce a first, negligible amount of source-draincurrent, illuminating the LET and applying a first source-drain voltagemay induce a second amount of source-drain current that is greater thanthe first source-drain current, and illuminating the LET and applying asecond source-drain voltage may induce a third amount of source-draincurrent that is greater than the second amount of source-drain current.The amount of source-drain current can then be interpreted to determinewhether the LET is illuminated and, if so, whether the appliedsource-drain voltage is the first source-drain voltage or the secondsource-drain voltage. If the first and second source-drain voltagesrepresent two distinct input states, the amount of source-drain currentcan therefore be interpreted to determine the input state.

In addition, two LET devices can be combined in parallel or series torespectively create universal NOR and NAND logic gates. NOR and NANDlogic gates are universal, which means that they can be used toconstruct most other logic circuits. Because a LET behaves similarly toan n-Metal-Oxide-Semiconductor FET (n-MOSFET), similar NOR and NANDlogic circuits and truth tables may be constructed as illustrated inFIGS. 6A-6D. Logic inversion is possible given the high resistivity ofan unilluminated LET (OFF state). In this case, the drive voltage,V_(DD), goes directly to the output port. For the parallel LET devicesin the NOR logic gate, FIG. 6A, illumination of either or both LETdevice(s), labelled as A and B, allows current to travel to ground, andyields an OFF state (“0”) at the output port. An ON state (“1”) is onlypossible when both LET devices are off or unilluminated. A NAND logiccircuit, FIG. 6B, on the other hand, contains two LET devices in series.Illumination of either LET device is insufficient for current to travelto ground, which results in an ON state at the output port. Illuminatingboth devices, however, produces an OFF state at the output port. Truthtables for both of these gates are shown in FIGS. 6C and 6D,respectively.

Referring back to FIGS. 5A-5H, as discussed above, LETs also offermulti-independent gate operation, where optical gates do not increasedevice dimensions. To appreciate this functionality, consider a scenarioin which a LET is illuminated with either uniform halogen illuminationor focused illumination from a 532 nm laser, denoted as P_(g1) andP_(g2), respectively. Illumination by either individual light beamproduces its corresponding transfer characteristics (as shown in FIGS.4A-4F). Illumination by both beams simultaneously, however, produces acurrent enhancement.

This effect is depicted in FIG. 5C, which is a contour plot showing thevalues of a current enhancement factor R achievable with a fixed V_(ds)of 5.0 V, where

$R = {\frac{I_{ds}\left( {P_{g\; 1},P_{g\; 2}} \right)}{{I_{ds}\left( P_{g\; 1} \right)} + {I_{ds}\left( P_{g\; 2} \right)}}.}$

The current enhancement factor R varies based upon the applied P_(g1)and P_(g2). FIGS. 5D-5G demonstrate dual-gate applications in threedifferent current enhancement regions.

FIG. 5D demonstrates the ability of a LET to perform opticalamplification. This can occur in the super-linear region 500 and whereR>>1. In FIG. 5D, R≈9-11. As shown in FIG. 5D, single beam illuminationinduces currents of ˜11 nA and ˜37 nA under 532-nm excitation at 2 nWand halogen illumination at 1.57 respectively. On the other hand,simultaneous illumination by the same two beams yields a current of ˜525nA. In other words, the simultaneous illumination yields a current thatis approximately 11 times greater than the sum of the currents inducedunder single beam illumination. If the laser beam is viewed as a weakoptical signal to be measured, and the halogen light is viewed as a gatesignal, an amplification factor of m=48 is obtained. This feature of aLET may find broad application in weak optical signal detection.

FIG. 5E demonstrates that, based on the results shown in FIG. 5D, a LETcan also be used for optical logic operations. For example, twoindividually applied optical gates, with inputs of A and B,respectively, produce two low current or OFF states represented as (1,0)or (0,1) (the (0,0) OFF state is not shown for clarity). Only undersimultaneous illumination does the device produce the ON or (1,1) state.Thus, when two beams are controlled based on two input states,respectively, the induced source-drain current can be interpreted todetermine whether both of the input states are ON states.

FIG. 5F demonstrates the ability of a LET to perform optical summation.This can occur in the linear region 502 and where R=1, for example. Asshown in FIG. 5F, single beam illumination induces currents of ˜2 μA and˜0.32 μA under 532-nm excitation at 0.63 μW and halogen illumination at0.7 μW, respectively. On the other hand, simultaneous illumination bythe same two beams yields a current of 2.43 μA or approximately theirnumerical sum. Thus, this region is convenient for producing multiplestates, such as for memory devices.

For example, without any illumination, the LET may produce a first,negligible level of current corresponding to a first state. Further,illumination by only the first beam may produce a second level ofcurrent corresponding to a second state, illumination by only the secondbeam may produce a third level of current corresponding to a thirdstate, and simultaneous illumination may produce a fourth level ofcurrent corresponding to a fourth state. A computing device cantherefore interpret a detected amount of current as being indicative ofone of the four states based on the detected amount of current beingclosest to a particular one of the four current levels.

FIG. 5G demonstrates the ability of a LET to be used as an optical ORlogic gate. This can occur in the sublinear/saturation region 504 andwhere R=½, for example. As shown in FIG. 5G, when A corresponds to532-nm excitation at 0.63 μW and B corresponds to halogen illuminationat 69.1 μW, individual illumination as (1,0) and (0,1) states or dualillumination as the (1,1) state all produce comparable I_(ds) values;all three ON states contrast the OFF state which induces pA-level I_(ds)(not shown for clarity).

A single LET could perform more complex logic functions concurrently bycombining V_(ds) control with dual optical gate ability, such as athree-terminal AND gate with output A×B×C, or with simultaneous AND andOR gates with A×(B|C) output. Truth tables for these logic operationsand their proposed symbols are provided in FIGS. 7A-7D. In FIGS. 7A-7D,three inputs are A=V_(ds), B=P_(g1), and C=P_(g2). Changing between thetwo logic functions simply requires altering both optical gate powersfrom the super-linear region to the sublinear/saturation region.

FIG. 5H demonstrates the ability of a LET to be used as a differentiatorunder zero or low P_(g) or as an optically-gated phase tuner as P_(g) isincreased. In particular, FIG. 5H shows I_(ds)(t) vs. V_(ds)(t) curvesfor different P_(g) values, where V_(ds)(t) is a sine wave modulatedwith an amplitude of 5.0 V and a DC offset to remove the negativeportion. The I_(ds)(t) curve exhibits a 90-degree phase delay withrespect to V_(ds)(t) when P_(g)=0 (or P_(g) is relatively low), whichindicates that the device functions as a differentiator by convertingthe sine wave into a cosine wave. Increasing P_(g) results in a tunablephase shift that gradually approaches zero, e.g., at P_(g)=2.6 μW. Thiseffect can be understood as changing the LET's impedance by varying thegate power.

FIG. 8 is a flow chart of an example optical gating method. Method 800shown in FIG. 8 presents an embodiment of a method that, for example,may be performed by one or more computing devices (or components of oneor more computing devices). Example devices or systems may be used orconfigured to perform logical functions presented in FIG. 8. In someinstances, components of the devices and/or systems may be configured toperform the functions such that the components are actually configuredand structured (with hardware and/or software) to enable suchperformance. In other examples, components of the devices and/or systemsmay be arranged to be adapted to, capable of, or suited for performingthe functions. Method 800 may include one or more operations, functions,or actions as illustrated by one or more of blocks 802-810. Although theblocks are illustrated in a sequential order, these blocks may also beperformed in parallel, and/or in a different order than those describedherein. Also, the various blocks may be combined into fewer blocks,divided into additional blocks, and/or removed based upon the desiredimplementation.

It should be understood that for this and other processes and methodsdisclosed herein, flowcharts show functionality and operation of onepossible implementation of present embodiments. In this regard, eachblock may represent a module, a segment, or a portion of program code,which includes one or more instructions executable by a processor forimplementing specific logical functions or steps in the process. Theprogram code may be stored on any type of computer readable medium ordata storage, for example, such as a storage device including a disk orhard drive. The computer readable medium may include non-transitorycomputer readable medium or memory, for example, such ascomputer-readable media that stores data for short periods of time likeregister memory, processor cache and Random Access Memory (RAM). Thecomputer readable medium may also include non-transitory media, such assecondary or persistent long term storage, like read only memory (ROM),optical or magnetic disks, compact-disc read only memory (CD-ROM), forexample. The computer readable media may also be any other volatile ornon-volatile storage systems. The computer readable medium may beconsidered a tangible computer readable storage medium, for example.

In addition, each block in FIG. 8 may represent circuitry that is wiredto perform the specific logical functions in the process. Alternativeimplementations are included within the scope of the example embodimentsof the present disclosure in which functions may be executed out oforder from that shown or discussed, including substantially concurrentor in reverse order, depending on the functionality involved, as wouldbe understood by those skilled in the art.

At block 802, the method 800 includes providing a photoconductive devicecomprising two metal contacts separated by a semiconductor channel. Forexample, the photoconductive device may be the LET 120 of FIG. 1 or aLET that is similar to the LET 120 of FIG. 1. In one example, thesemiconductor channel may be a semiconductor nanostructure.

At block 804, the method 800 includes controlling, based on a firstinput state, illumination of the semiconductor channel by an opticalbeam during a time period. In line with the discussion above, theoptical beam may be provided using a laser configured to provide anoptical beam with a particular wavelength and/or power level. Theoptical beam may be focused through one or more lenses at a particulararea of the semiconductor channel. Alternatively, the optical beam maybe provided using a light (e.g., a broad-band light source like halogenor other type of light like a single wavelength laser) configured toprovide uniform illumination of the semiconductor channel. Depending onthe desired configuration, the particular wavelength and/or power levelof the optical beam may be selected in order to cause thephotoconductive device to operate in a desired operating region (e.g., asuper-linear region, a linear region, or a sublinear/saturation region).

Controlling illumination based on the first input state may involveilluminating the semiconductor channel if the first input state is an ONstate but not illuminating the semiconductor channel if the first inputstate is an OFF state.

At block 806, the method 800 includes controlling, based on a secondinput state, a bias voltage applied across the two metal contacts duringthe time period. In line with the discussion above, the bias voltage maybe a source-drain voltage. Controlling the bias voltage based on thesecond input state may involve applying a high voltage (e.g., 5 V) ifthe second input state is an ON state but applying a low voltage (e.g.,0 V) if the second input state is an OFF state. Alternatively,controlling the bias voltage based on the second input state may involveapplying a first voltage (e.g., 0 V) if the second input state is an OFFstate, applying a second voltage (e.g., 5 V) if the second input stateis a low-ON state, and applying a third voltage (e.g., 14 V) if thesecond input state is a high-ON state).

At block 808, the method 800 includes detecting an amount of currentproduced by the photoconductive device during the time period. Forexample, the current may be detected using a current detector (e.g., alow voltage sourcemeter) having two electrical leads connected to thetwo metal contacts of the photoconductive device. In some instances, acurrent pre-amplifier may be used to amplify the current.

At block 810, the method 800 includes, based on the detected amount ofcurrent, providing an output indicative of the first input state and thesecond input state. In one example, the output may be indicative ofwhether the first and the second input states are both ON states. Forinstance, a computing device may be configured to determine whether thedetected amount of current is greater than a threshold amount ofcurrent. If the computing device determines that the detected amount ofcurrent is greater than the threshold amount, the computing device mayprovide an output indicative of the first and second input states bothbeing ON states. Whereas, if the computing device determines that thedetected amount of current is not greater than the threshold amount, thecomputing device may provide an output indicative of one or both of thefirst and second input states being an OFF state.

In another example, the computing device may provide an outputindicative of the first input state and the second input state dependingon which of three predetermined current levels the detected amount ofcurrent is closest to. For instance, the first input state may be eitheran ON input state or an OFF input state, and the second input state maybe either an OFF state, a low-ON state, or a high-ON state. If thedetected amount of current is closest to a first current level, thecomputing device may provide an output indicative of the first inputstate and the second input state both being OFF states. If the detectedamount of current is closest to a second current level that is greaterthan the first current level, the computing device may provide an outputindicative of the first input state being an ON state and the secondinput state being a low-ON state. And if the detected amount of currentis closest to a third current level that is greater than the secondcurrent level, the computing device may provide an output indicative ofthe first input state being an ON state and the second input state beinga high-ON state. The second current level and the third current levelcould, for example, correspond to the first and second plateaus visiblein FIG. 3B.

FIG. 9 is a flow chart of another example optical gating method. Method900 shown in FIG. 9 presents an embodiment of a method that, forexample, may be performed by one or more computing devices (orcomponents of one or more computing devices).

At block 902, the method 900 includes providing a photoconductive devicehaving a metal-semiconductor-metal structure. For example, thephotoconductive device may be the LET 120 of FIG. 1 or a LET that issimilar to the LET 120 of FIG. 1. Accordingly, the photoconductivedevice may include two metal contacts separated by a semiconductorchannel (e.g., a semiconductor nanostructure).

At block 904, the method 900 includes applying a bias voltage to thephotoconductive device. For example, the bias voltage may be appliedacross two metal contacts of the photoconductive device.

At block 906, the method 900 includes controlling, based on a firstinput state, illumination of the photoconductive device by a firstoptical beam during a time period. In line with the discussion above,the first optical beam may be provided using a laser configured toprovide an optical beam with a particular wavelength and/or power level.The first optical beam may be focused through one or more lenses at aparticular area of the semiconductor nanostructure channel.Alternatively, the first optical beam may be provided using a light(e.g., a broad-band light source like halogen or other type of lightlike a single wavelength laser) configured to provide uniformillumination of the semiconductor nanostructure channel. Depending onthe desired configuration, the particular wavelength and/or power levelof the first optical beam may be selected in order to cause thephotoconductive device to operate in a desired operating region (e.g., asuper-linear region, a linear region, or a sublinear/saturation region).

Controlling illumination based on the first input state may involveilluminating the semiconductor channel using the first optical beam ifthe first input state is an ON state but not illuminating thesemiconductor channel using the first optical beam if the first inputstate is an OFF state.

At block 908, the method 900 includes controlling, based on a secondinput state, illumination of the photoconductive device by a secondoptical beam during the time period. The second optical beam may beprovided using a laser configured to provide an optical beam with aparticular wavelength and/or power level. The second optical beam may befocused through one or more lenses at a particular area of thesemiconductor nanostructure channel. Alternatively, the second opticalbeam may be provided using a light (e.g., a halogen or other type oflight) configured to provide uniform illumination of the semiconductornanostructure channel. Depending on the desired configuration, theparticular wavelength and/or power level of the second optical beam maybe selected in order to cause the photoconductive device to operate in adesired operating region (e.g., a super-linear region, a linear region,or a sublinear/saturation region).

Controlling illumination based on the second input state may involveilluminating the semiconductor channel using the second optical beam ifthe second input state is an ON state but not illuminating thesemiconductor channel using the second optical beam if the second inputstate is an OFF state.

In one example, the first optical beam and the second optical beam canbe combined into one beam but controlled independently. With thisapproach, the first optical beam and the second optical beam may bedirected at substantially the same area of the semiconductornanostructure. In other examples, the first optical beam and the secondoptical beam may be directed at substantially different areas of thesemiconductor nanostructure.

At block 910, the method 900 includes detecting an amount of currentproduced by the photoconductive device during the time period. Forexample, the current may be detected using a current detector (e.g., alow voltage sourcemeter) having two electrical leads connected to thetwo metal contacts of the photoconductive device. In some instances, acurrent pre-amplifier may be used to amplify the current.

At block 912, the method 900 includes, based on the detected amount ofcurrent, providing an output indicative of the first input state and thesecond input state. In some instances, the output may be indicative of alogical combination of the first input state and the second input state.For instance, the output may be indicative of whether the first and thesecond input states are both ON states. To provide such an output, thecomputing device may be configured to determine whether the detectedamount of current is greater than a threshold amount of current. If thecomputing device determines that the detected amount of current isgreater than the threshold amount, the computing device may provide anoutput indicative of the first and second input states both being ONstates. Whereas, if the computing device determines that the detectedamount of current is not greater than the threshold amount, thecomputing device may provide an output indicative of one or both of thefirst and second input states being an OFF state.

In another configuration, the output may be indicative of whether thefirst or the second input state is an ON state. To provide such anoutput, the computing device may be configured to determine whether thedetected amount of current is greater than a threshold amount ofcurrent. If the computing device determines that the detected amount ofcurrent is greater than the threshold amount, the computing device mayprovide an output indicative of one or both of the first and secondinput states being ON states. Whereas, if the computing devicedetermines that the detected amount of current is not greater than thethreshold amount, the computing device may provide an output indicativeof both the first and second input states being an OFF state.

In other examples, the computing device may provide an output indicativeof the first input state and the second input state depending on whichof a plurality of predetermined current levels that the detected amountof current is closest to. For instance, both the first input state andthe second input state may either be ON states or OFF states, and thecomputing device may provide a particular output depending on which offour predetermined current levels the detected amount of current isclosest to. If the detected amount of current is closest to a firstcurrent level, the computing device may provide an output indicative ofthe first input state and the second input state both being OFF states.If the detected amount of current is closest to a second current levelthat is greater than the first current level, the computing device mayprovide an output indicative of the first input state being an ON stateand the second input state being an OFF state. If the detected amount ofcurrent is closest to a third current level that is greater than thesecond current level, the computing device may provide an outputindicative of the first input state being an OFF state and the secondinput state being an ON state. And if the detected amount of current isclosest to a fourth current level that is greater than the third currentlevel, the computing device may provide an output indicative of both thefirst input state and the second input state being an ON state. The fourcurrent levels could, for example, correspond to the four current levelsvisible in FIG. 5F. In other examples, there may be more or lesspredetermined current levels to which the detected amount of current iscompared, depending on the particular configuration.

In some examples, the method 900 may further include controlling, basedon a third input state, the bias voltage applied across the two metalcontacts during the time period. Controlling the bias voltage based onthe third input state may involve applying a high voltage (e.g., 5 V) ifthe third input state is an ON state but applying a low voltage (e.g., 0V) if the third input state is an OFF state. Further, as discussed abovewith reference to FIG. 7, the output at block 912 may then be indicativeof a logical combination of the first input state, the second inputstate, and the third input state. For instance, the output mayindicative of whether all three of the input states are ON states. Orthe output may indicative of whether the third input state is an ONstate and one or both of the first and second input states are ONstates.

FIG. 10 is a flow chart of an example method for optical amplification.Method 1000 shown in FIG. 10 presents an embodiment of a method that,for example, may be performed by one or more computing devices (orcomponents of one or more computing devices).

At block 1002, the method 1000 includes providing a photoconductivedevice having a metal-semiconductor-metal structure. For example, thephotoconductive device may be the LET 120 of FIG. 1 or a LET that issimilar to the LET 120 of FIG. 1. Accordingly, the photoconductivedevice may include two metal contacts separated by a semiconductorchannel (e.g., a semiconductor nanostructure).

At block 1004, the method 1000 includes applying a bias voltage to thephotoconductive device. For example, the bias voltage may be appliedacross two metal contacts of the photoconductive device.

At block 1006, the method 1000 includes illuminating the photoconductivedevice using an optical beam while simultaneously exposing thephotoconductive device to an optical signal during the time period. Theoptical beam may be provided using a laser configured to provide anoptical beam with a particular wavelength and/or power level. Theoptical beam may be focused through one or more lenses at a particulararea of the semiconductor channel. Alternatively, the optical beam maybe provided using a light (e.g., a halogen or other type of light)configured to provide uniform illumination of the semiconductor channel.The particular wavelength and/or power level of the optical beam may beselected in order to cause the photoconductive device to operate in adesired operating region (e.g., a super-linear region). In someexamples, the optical beam and the optical signal may be in differentwavelength regions. For instance, one of the optical beam and theoptical signal may be infrared and the other may be visible, one of theoptical beam and the optical signal may be visible and the other may beultraviolet, etc.

Exposing the photoconductive device to the optical signal may includeexposing the semiconductor channel to the optical signal. In someexamples, the optical signal may be an infrared signal. The opticalsignal may be focused (e.g., through one or more lenses) to a particulararea of the semiconductor channel. Alternatively, the optical signal mayilluminate the whole semiconductor channel.

At block 1008, the method 1000 includes detecting an amount of currentproduced by the photoconductive device during the time period. Forexample, the current may be detected using a current detector (e.g., alow voltage sourcemeter) having two electrical leads connected to thetwo metal contacts of the photoconductive device. In some instances, acurrent pre-amplifier may be used to amplify the current.

In line with the discussion above, advantageously, illuminating thephotoconductive device using the optical beam may amplify the amount ofcurrent induced by the optical signal by a factor of at least 10. Insome instances, illuminating the photoconductive device using theoptical beam may amplify the amount of current induced by the opticalsignal by a factor of at least 40.

In some examples, the method 1000 may further include applying a biasvoltage across the two metal contacts while illuminating thephotoconductive device using the optical beam and simultaneouslyexposing the photoconductive device to the optical signal.

In one configuration, the photoconductive device may include an array ofLETs. Further, the array of LETs may be illuminated with uniform,broad-area illumination while exposing the array of LETs to an opticalsignal. The optical signal may pass through one or more lenses or otheroptical components which direct portions of the optical signal toindividual LETs of the array. The computing device could then beconfigured to detect individual current levels induced in each of theLETs of the array, and to process the individual current levels tocreate an image. With this approach, the detected amount of current fromeach LET of the array may be proportional to an intensity of the opticalsignal. Other configurations are also possible.

As mentioned, portions of the methods 800, 900, and 1000 may beperformed by one or more computing devices (or components of one or morecomputing devices). FIG. 11 is a schematic diagram of an examplecomputing device 1100. In some examples, some components illustrated inFIG. 11 may be distributed across multiple computing devices. However,for the sake of example, the components are shown and described as partof one example device 1100. The computing device 1100 may be or includea mobile device, desktop computer, email/messaging device, tabletcomputer, or similar device that may be configured to perform thefunctions described herein.

As shown in FIG. 11, the computing device 1100 may include one or moreprocessors 1102, a memory 1104, a communication interface 1106, adisplay 1108, and one or more input devices 1110. Components illustratedin FIG. 11 may be linked together by a system bus, network, or otherconnection mechanism 1112. The computing device 1100 may also includehardware to enable communication within the computing device 1100 andbetween the computing device 1100 and another computing device (notshown), such as a server entity. The hardware may include transmitters,receivers, and antennas, for example.

The one or more processors 1102 may be any type of processor, such as amicroprocessor, digital signal processor, multicore processor, etc.,coupled to the memory 1104. The memory 1104 may be any type of memory,such as volatile memory like random access memory (RAM), dynamic randomaccess memory (DRAM), static random access memory (SRAM), ornon-volatile memory like read-only memory (ROM), flash memory, magneticor optical disks, or compact-disc read-only memory (CD-ROM), among otherdevices used to store data or programs on a temporary or permanentbasis.

Additionally, the memory 1104 may be configured to store programinstructions 1114. The program instructions 1114 may be executable bythe one or more processors 1102. For instance, the program instructions1114 may cause the one or more processors 1102 to perform any of thefunctions of methods 800, 900, and 1000, or any of the functionsdescribed herein.

The communication interface 1106 may be configured to facilitatecommunication with one or more other devices, in accordance with one ormore wired or wireless communication protocols. For example, thecommunication interface 1106 may be configured to facilitate wirelessdata communication for the computing device 1100 according to one ormore wireless communication standards, such as one or more IEEE 802.11standards, ZigBee standards, Bluetooth standards, etc. As anotherexample, the communication interface 1106 may be configured tofacilitate wired data communication with one or more other computingdevices, such as one or more cloud-connected computing devices orservers.

The display 1108 may be any type of display component configured todisplay data. As one example, the display 1108 may include a touchscreendisplay. As another example, the display may include a flat-paneldisplay, such as a liquid-crystal display (LCD) or a light-emittingdiode (LED) display.

The one or more input devices 1110 may include one or more pieces ofhardware equipment used to provide data and control signals to thecomputing device 1100. For instance, the one or more input devices 1110may include a mouse or pointing device, a keyboard or keypad, amicrophone, a touchpad, a touchscreen, or a camera, among other possibletypes of input devices.

The description of the different advantageous arrangements has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. Further, different advantageousembodiments may provide different advantages as compared to otheradvantageous embodiments. The embodiment or embodiments selected arechosen and described in order to best explain the principles of theembodiments, the practical application, and to enable others of ordinaryskill in the art to understand the disclosure for various embodimentswith various modifications as are suited to the particular usecontemplated.

What is claimed is:
 1. A method comprising: providing a photoconductivedevice having a metal-semiconductor-metal structure; controlling, basedon a first input state, illumination of the photoconductive device by afirst optical beam during a time period; controlling, based on a secondinput state, illumination of the photoconductive device by a secondoptical beam during the time period; detecting an amount of currentproduced by the photoconductive device during the time period; and basedon the detected amount of current, providing an output indicative of thefirst input state and the second input state.
 2. The method of claim 1:wherein the photoconductive device comprises two metal contactsseparated by a semiconductor channel, wherein controlling illuminationof the photoconductive device by the first optical beam comprisescontrolling illumination of the semiconductor channel by the firstoptical beam, and wherein controlling illumination of thephotoconductive device by the second optical beam comprises controllingillumination of the semiconductor channel by the second optical beam. 3.The method of claim 2, wherein detecting the amount of current producedby the photoconductive device comprises detecting an amount of currentflowing between the two metal contacts.
 4. The method of claim 2,wherein the semiconductor channel comprises a semiconductornanostructure channel.
 5. The method of claim 2, further comprising:controlling, based on a third input state, a bias voltage applied acrossthe two metal contacts during the time period, wherein the output isindicative of a logical combination of the first input state, the secondinput state, and the third input state.
 6. The method of claim 2,wherein the first optical beam and the second optical beam are directedat substantially the same area of the semiconductor channel.
 7. Themethod of claim 2, wherein the first optical beam and the second opticalbeam are directed at substantially different areas of the semiconductorchannel.
 8. The method of claim 2, wherein the illumination of thesemiconductor channel by the first optical beam comprises focusedillumination of a particular area of the semiconductor channel, andwherein the illumination of the semiconductor channel by the secondoptical beam comprises uniform illumination of the semiconductorchannel.
 9. The method of claim 1, wherein the output is indicative of alogical combination of the first input state and the second input state.10. The method of claim 9, wherein providing, based on the detectedamount of current, the output indicative of the first input state andthe second input state comprises: providing an ON output if the detectedamount of current is greater than a threshold current, wherein the ONoutput is indicative of the first input state and the second input stateboth being ON states; and providing an OFF output if the detected amountof current is not greater than the threshold current, wherein the OFFoutput is indicative of one or both of the first input state and thesecond input state being an OFF state.
 11. The method of claim 9,wherein providing, based on the detected amount of current, the outputindicative of the first input state and the second input statecomprises: providing an ON output if the detected amount of current isgreater than a threshold current, wherein the ON output is indicative ofone or both of the first input state and the second input state being ONstates; and providing an OFF output if the detected amount of current isnot greater than the threshold current, wherein the OFF output isindicative of both the first input state and the second input statebeing an OFF state.
 12. The method of claim 1, wherein providing, basedon the detected amount of current, the output indicative of the firstinput state and the second input state comprises providing a particularone of at least four output states based on the detected amount ofcurrent being closest to a particular one of at least four respectivepredetermined current levels.
 13. The method of claim 1: whereincontrolling, based on the first input state, the illumination of thephotoconductive device by the first optical beam comprises illuminatingthe photoconductive device with the first optical beam if the firstinput state is an ON input state but not illuminating thephotoconductive device with the first optical beam if the first inputstate is an OFF input state, and wherein controlling, based on thesecond input state, the illumination of the photoconductive device bythe second optical beam comprises illuminating the photoconductivedevice with the second optical beam if the second input state is an ONinput state but not illuminating the photoconductive device with thesecond optical beam if the second input state is an OFF input state. 14.A method for optical amplification, comprising: providing aphotoconductive device having a metal-semiconductor-metal structure;illuminating the photoconductive device using an optical beam whilesimultaneously exposing the photoconductive device to an optical signalduring a time period; and detecting an amount of current produced by thephotoconductive device during the time period; wherein illuminating thephotoconductive device using the optical beam amplifies the amount ofcurrent induced by the optical signal.
 15. The method of claim 14:wherein the photoconductive device comprises two metal contactsseparated by a semiconductor channel, wherein illuminating thephotoconductive device using the optical beam comprises illuminating thesemiconductor channel using the optical beam, and wherein exposing thephotoconductive device to the optical signal comprises exposing thesemiconductor channel to the optical signal to be measured.
 16. Themethod of claim 15, further comprising applying a bias voltage acrossthe two metal contacts while illuminating the photoconductive deviceusing the optical beam and simultaneously exposing the photoconductivedevice to the optical signal, wherein illuminating the photoconductivedevice using the optical beam amplifies the current induced by theoptical signal by a factor of at least
 10. 17. The method of claim 14,wherein exposing the photoconductive device to the optical signalcomprises focusing the optical signal to a particular area of thesemiconductor channel.
 18. A method comprising: providing aphotoconductive device comprising two metal contacts separated by asemiconductor channel; controlling, based on a first input state,illumination of the semiconductor channel by an optical beam during atime period; controlling, based on a second input state, a bias voltageapplied across the two metal contacts during the time period; detectingan amount of current produced by the photoconductive device during thetime period; and based on the detected amount of current, providing anoutput indicative of the first input state and the second input state.19. The method of claim 18, wherein providing, based on the detectedamount of current, the output indicative of the first input state andthe second input state comprises: providing an ON output if the detectedamount of current is greater than a threshold current, wherein the ONoutput is indicative of the first input state and the second input stateboth being ON states; and providing an OFF output if the detected amountof current is not greater than the threshold current, wherein the OFFoutput is indicative of one or both of the first input state and thesecond input state being an OFF state.
 20. The method of claim 18,wherein providing, based on the detected amount of current, the outputindicative of the first input state and the second input statecomprising: providing a first output state if the detected amount ofcurrent is closest to a first one of three predetermined current levels,wherein the first output state is indicative of both the first inputstate and the second input state being OFF states; providing a secondoutput state if the detected amount of current is closest to a secondone of the three predetermined current levels, wherein the second outputstate is indicative of the first input state being an ON input state andthe second input state being a low-ON state; and providing a thirdoutput state if the detected amount of current is closest to a third oneof the three predetermined current levels, wherein the third outputstate is indicative of the first input state being an ON input state andthe second input state being a high-ON state.